Carrier-current system



May 5, 1925.

J. MILLS CARRIER CURRENT SYSTEM Filed Feb. 27, 1920 In venfor JohnM/'//s Arry Patented May 5, 1925.

UNITED STATES. PATENT OFFICE.

JOHN MILLS, OF WYOMING, NEW JERSEY, ASSIGNOR T0 WESTERN ELECTRIC COM-PANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

CARRIER-CURRENT SYSTEM.

. Application filed February 27', 1920. Serial No.'361,911.

To all whom it may concern:

Be it known that I, JoHNMILLs, acitizen of the United States, residingat Wyoming, in the county of Essex, State of New Jersey, have inventedcertain new and useful Improvements in Carrier-Current Systems, of whichthe following is a full, clear, concise, and exact description.

This invention relates to transmission systems and more particularly toduplex or multiplex systems of the carrier current. type for duplexcommunication between a plurality of pairs of stations.

The invention is illustrated in connection with a multiplex carriercurrent system comprising a number of duplex channels, each channelconnecting a pair of telephone substations between which transmission isto be carried on. The duplex channels of the system disclosed have incommon a high frequency four-wire circuit of which one pair ofconductors serves to transmit all the messages in one direction and ofwhich another pair serves to transmit all the me; sages in the otherdirection.

Circuits of this type frequently rely upon a condition of balancebetween the real line and an artificial net-work of impedance elements.The currents which are transmitted in one direction are prevented bythis balanced arrangement from being transmitted back to the sending endthrough the oppositely directed repeater and the other part of thefour-wire. circuit. Due to changes in weather conditions or othercauses, it is sometimes difficult or impracticable to maintain thiscondition or balance, and as aresult there are heard echoes which lagslightly behind the original voice sounds.

' It is one object of the invention to provide means for preventing suchechoes.

In the form of the invention disclosed, a pair of normally openedswitches are introduced into a circuit connected to the o tgoing andreturn portions espectively of each du lex channel. Upon being closed.thcse switches short-circuit the conductor to which they are connected.Each switch is so arrang so; as to be closed by a relay asso- :iatcdwith the other branch of the duplex chmmeh The current, to operate therolavs may consist of or be derived from the high frequency modulatedcurrents or in the case of a terminal modulating station may be of lowfrequency.

T he mvention therefore contemplates the provision of means such as aline balancing net-work designed to prevent reflection currents in therepeater circuit and additional means to prevent their How in case thefirst means is inadequate. Conversely, if the additional means shouldbecome inoperative, the circuit is not rendered inoperative since, asstated above, the switch contactsaie normally opened.

An important feature of the invention consists in means whereby in aduplex-multiplex system, the transmission of current in one directionthrough one duplex channel serves to temporarily render the returnchannel inoperative or greatly reduce its efiiciency without interferingwith either the going or return paths of any of the other duplexchannels. Since in particular installations reasons other than theprevention of echoing or round trip singing may exist for making itdesirable to accomplish this result, the invention is'not limited tothis purpose alone but may be advantage ously utilized wherever it isdesired that onehalf of a duplex channel act at certa times upon thehalf of the channel which transmits in the opposite direction withoutadversely affecting the other half of the channel or t other channels orimpairing their transmitting etiiciency. While -ow frequency hybridcoils are herein described as the elements causing reflection currentsitshould be understood it is not contemplated tlr. the invention islimited to the prevention of reflection currents produced by suchelements. Such reflection currents, as is well known may be produced byline trans formers, by repeater elements, by termina apparatus, byabnormally irregular iine loading, or by connection of line sections ofdifferent impedance characteristics to each other-. Furthermore. asstated above, it is not contemplated that. the invention is limited tothe prevention be utilized for oth r purposes.

From another a. wet. the imrr-minn (mnot echoing z ltn'iay i prises arepeater having several pairs of one-way channels, one or both of thechannels of a pair being so related to the other that the existence of acertain condition in one of the channels produces a variation from thenormal condition in the other channel without affecting the condition ofany other channel of the remaining pairs of channels.

In the preferred form means are included whereby current of a particularrange of frequencies only is allowed to act upon the devices forrendering inoperative a given channel, currents of other frequenciesbeing excluded.

For a detailed description, reference will be had to the accompanyingdiagram wherein Fig. 1 shows the features of the invention applied to amultiplex system adapted to allow communication between several pairs oflow frequency stations over the same four-wire circuit; and Fig. 2 is acircuit diagram showing the arrangement of parts at one end only of thehigh frequency line in a modified form of the invention.

\Vhere? the expression band of frequencies.-or range of frequencies isused herein, it is meant to include a range which may be limited to asingle frequency as well as a band comprising a plurality offrequencies.

Referring to Fig. l, the circuits of the system will be traced throughin one direction from a substation S to a substation S which will serveequally for tracing the circuits in the opposite direction, the partsoccurring in the same order, having the same functions, and beingdesignated by identical reference characters. Beginning with substationS, this station is connected to the hybrid coil HC, through the low'frequency line L1). The line LL may include one or more telephoneexchanges or repeater stations if desired and is intended to indicateany line through which low frequency signal currents may be propagatedbut in the present instance is considered as a telephone line. The lineLL is balanced by the network LN. From hybrid coil HP to the modulator Mconnection is made through a circuit 8. The modulator is supplied withcarrier frequency current by a suitable generator G or other suitablesource. The modulator which may be of any known type of carriersuppression modulator functions to modulate the current supplied by G inaccordance with the signaling currents transmitted through conductor 8.The modulated currents are amplified by amplifier A and pass throughfilter F, for impression upon the line it) of a four-wire circuit ll.Modulated currents of other carrier frequencies from other low frequencylines are also impressed upon the line 10 through a series of otherfilters of which F only is shown.

At a repeater station, the incoming currents are amplified by amplifierA and impressed upon band filters BF BF etc. These filters are eachadapted to'transmit only the band of frequencies corresponding to onemodulated high frequency current. The filter BF for example, transmitsthe current whose base frequency corresponds to that of the generator G.After passing the filter BF a small portion of the current is utilizedin actuating the thermionic amplifying relay TR whose input circuitincludes the high resistance r in shunt to the line and whose outputcircuit contains an alternating currentelectromagnetic relay R which isadapted to close the switch S. The thermionic relay however, may be connected to the line in other ways. The switch S is located in a circuit12 which is in shunt to the return channel of the repeater station whichis adapted to transmit messages from the station S to station S. lVhenthe switch S is closed, this return channel is short-circuited andrendered inoperative. The principal part of the high frequency currentafter passing filter BF passes through a similar band filter B1 andcontinues on through the line 10 to the detector station where thedesired frequency is selected from the other modulated waves by thefilter F detected by detector 1) and amplitied by the amplifier A. Thedetected currents pass through the conductor 9 to the hybrid coil HC andthe line LL to the station S.

Associated with the other channel at the repeater station is a modifiedform of apparatus connected between the band filters BF and BF which aredesignedto selectively transmit other modulated currents, as forexample, those passing through the filter F Tn this form a relay isincluded directly in the line and actuatcs the switch S to close thecircuit 12. Any form of apparatus other than the two shown herein may bedesigned to accomplish the same function.

() wrafion.

\Ylicn messages are transmitted from station S to station S, there willsometimes be a reflection or transmission of energy through the returnchannel owing to an unbalance 0t theline LL and its network LN adjacentto station S. These currents being reverscly transmitted wouldordinarily 'be repeated and, amplified by the various apparatus alongthe line, heard by the operator at S as echoes. \Vhcn the improvementsherein described are used howercr, the return channel will beshortcircuitcd so that this echoing will not occur. The relay lt mayhave a time lag approximately equal to but preferal ily somewhat lessthan the time of transmission of a wave from the repeater station to thereflection point and back again. This CH- cumstancc may be utilized indesigning the relays. The time lag may be as much less than this amountas desired. Since, however, it is desired that: the return channel berestored to normal condition as quickly as possible, it is advantageousthat these relays be of the quick release type. It should be noted thatin this system the modulators M are of the carrier suppression typewhich cause the transmission of modulated current only during the timethe low frequency waves are acting thereon. The principles of theinvention are applicable to other systems in which carrier suppressionmodulators are not used.

If desired, the generators G which supply the out-going and incomingcarrier frequencies of a single duplex channel, may generate the same ordifferent frequencies. same frequency is employed in both direc tionsthe filters F, may be identical or they may be so designed that onefilter passes the lower side band p-s and the other the upper side band[7+8 where represents the carrier frequency in cycles per second and 3represents the signaling frequency. In the latter case, the frequencieswill preferably be staggered, that is, the frequencies of one side bandtransmitted in one direction will be intermediate to other bandstransmitted in the opposite direction. Inthe case where the generatorshave different -trequcimies, staggering may also be used; thus from oneendhot' the line. transmission may be carried on upon carrier waveshaving frequencies of 10,000, 20,000, etc., and in other directions.upon waves having carrier frequencies of 15,000, 25,000, etc.

Referring to Fig. 2 there will beseen at the station S the line LL. the.hybrid coil HC, circuit 8, generator modulator M, amplifier A, and thefilters F,, F, and F arranged in a manner similar to that in Fig. 1. Tothe circuit 8, however, is connected in any suitable manner a relaycircuit 12 containing any suitable device such as relayH' arranged, uponactuation, to close a switch S, and thereby short-circuit the returnchannel leading to the input side of the detector D. At the modulatingand detecting station at the remote .end of the line (not shown), whichis preferably identical with that described, another relay R and switchS, are provided. They are preferably to be. con nected, however, so thatthe relay will be actuated by currents traversing the system,

in the opposite'direction to those actuating 'relay S, and will renderinoperative the return path corresponding thereto. The switch S, may beconnected to the output circuit of the detector or in any other way tothe return path so as-to render it inoperative. It will be understoodthat the filters F and F, will each be connected to another hybrid coilHC and low frequency line LL, and the filters F5 and F will likewise beconnected If the to still another ybrid coil, etc. The operation of thissyst am will be fully understood by comparison with the description ofthe operation of no system shown in Fig. l.

The subject-matter of this specification may be regarded as animprovement in some respects upon that described in United States PatentNo. Lat-34,790, issued Xovember 7, 1922, Serial No. 322,154 filedSeptember 0, 1919.

lVhile certain complete systems embodying the invention are describedherein, the invention is not limited to a particular embodiment but maybe embodied in other constructions. Certain features may be utilized inother systems. The novel features which are believed to be inherent inthe invention are defined in the appended claims.

- What is claimed is:

1. A system for duplex-multiplex transmission comprising pairs ofstations, means for accomplishing a plurality of two-way transmissionson different frequency-selective. channels between pairs ofcommunicating stations, and means associate .l-with said system wherebyone-way transmission between one of said pairs of stations renderstransmission in the opposite direction impossible, said last mentionedmeans functioning without interfering with transmission between othersof said pairs of stations.

2. A system of duplex-multiplex signaling having aplurality of channels,each of which has separate outgoing and incoming paths, an echopreventing means in each path of a plurality of said channels operliraable under the influence of waves trans,

mitted in onedirection by one of said paths to render the cooperatingoppositely-conducting path incapable of efficient transmisslon.

3. A telephone system comprising means for simultaneously connectingseveral pairs of stations, said means comprising common conductive linesections, reflection produc ing means in said system, and meanscontrolled by signal currents for reventing reflection currents due tosigna s passing between one of said pairs of stations from retraversingthe line, said means functioning without destroying the transmittingefficiency of said system asto other pairs of stations.

4. A carrier current system comprising a tour-wire circuit, means fortransmitting several highfrequency modulated currents ineach directionover said circuit, reflection causing means in said circuit, and meansoperable by one of .said modulated currents passing through said circuitto rendersaid circuit incapable of transmitting reflection currents dueto said modulated current.

v5.'A duplex four-wire circuit, means for transmitting a plurality ofhigh frequency modulated currents through said circuit, and meanswhereby transmission in one direction of said currents renders saidsystem incapable of transmitting in the-"other direction until thecessation of transmission.

6. A repeater circuit comprising several sets of selective circuits forthe transmission of selected bands of frequencies in each direction,means operable by a small fraction of the energy being transmittedthrough any one of a plurality of said circuits to render another ofsaid circuits temporarily inoperative for transmission in the oppositedirection.

7. A four-wire circuit for duplex-multiplex carrier transmission,comprising reflection causing means at each terminal, a two-way repeaterin said circuit, said repeater comprising individual circuits forselectively repeating different signaling waves, and a relay associatedwith each one-way circuit for rendering the one-way circuit fortransmitting in the opposite directioninefiicient, said relay having atime lag substantially equivalent to the time of transmission. of a wavefrom said repeater to a reflection point and return.

8. A multiplex transmission systemhaving a channel pointed in onedirection, a channel pointed in the other direction and means forrendering said system inoperative for the transmission of a certainbandof frequencies in one direction, said means comprising two filtersin said channel pointed in one direction, and a device for controllingthe channel pointed in the other direction operatively associated withsaid first named channel between said filters and shielded thereby fromthe portions of the system lying beyond said filters.

t). A repeater normally adapted to repeat more than two bands offrequencies, each hand reprcsenting a signaling channel, devicesincluding relay means operative under the influence of electric currentsfor rendcring said repeater inoperative to repeat a certain band offrequencies, and aselective circuit for causing currents of a limitedfrequency range only to operate said devices.

10. A transmission system for transmitting several bands of frequenciesin one direction and several bands of frequencies in the otherdirection, devices operative under the influence of a band individual toeach device and selected from several bands transmitted in one directionto render said system inoperative to transmit a certain band offrequencies in one direction, and a selective circuit to limit the rangeof frequencies of currents affecting said devices.

11. A repeating station comprising means for repeating several bands offrequencies in one direction and means for repeating corresponding bandsof frequencies in the other direction, and means operative by repeatingany one of said bands to render the means for repeating theeorrseponding band in the other direction inoperative.

12. A repeater. system having several duplex channels and meanscooperating with each duplex channel whereby the transmission of wavestherethrough in one direction changes from efficient transmitting toineflicient transmitting condition'the channel for repeating waves inthe other direction.

13. A repeater system having several means each adapted to accomplishthe repeating of a particular band of frequencies in one direction,several means each adapted to accomplish the repeating of a particularband of frequencies in another direction, and means whereby thetransmission of any selected band of said bands of frequencies in onedirection renders the system less efficient for the transmission of aselected hand of frequencies in the other direction.

14. A multiplex signaling system including channels for the transmissionof messages between several pairs of substations, said channelscomprising conductors in common, and means operated by message currentsin one direction in one of said channels for reducing the transmittingefficiency of the return path of said channel without causing materialinterference with any other of said channels.

15. A multiplex repeater circuit comprising several sets of selectivecircuits for the transmission of selected bands of frequencies, eachcomprising a speech modulated carrier wave, in each direction, and meansoperable by a fraction of the energy being transmitted through one ofsaid circuits to render one of said circuits adapted for transmission inthe opposite direction temporarily inoperative.

16. A repeater circuit comprising several sets of selective circuits forthe transmission of selected bands of frequencies in each direction, oneof said bands in each direction representing a telephone message, andmeans operable by the transmission of one of said bands comprisingtelephone message energy through one of said channels in one directionto render inoperative a telephone message channel for transmission inthe opposite direction.

17. A transmission system for transmitting several bands of frequenciesin one direction and several bands of frequencies in the otherdirection, a plurality of said bands in each direction representing acor-responding plurality of telephone messages, devices operative underthe influence of transmission of one of said bands in one direction torender said system inoperative to transmit a certain band of frequenciesllil) lit) in one direction, and a selective circuit to other direction,wave controlled means operative by repeating one ofsaid bands to render.the means for repeating a corresponding band in the other directioninoperative, and a band pass filter for-limiting to a definite range offrequencies the trans.- fer of currents to said wave controlled means.

19. A. telephone transmission system having several duplex telephonetransmission channels, means cooperating with each duplex channelwhereby the transmission of waves therethrough in one direction variesthe normal condition of the channel for repeating waves in the otherdirection, and means preventing a corresponding variation of conditionof another duplex channel.

20. A transmission system having several duplex channels, eachcomprising frequency-selective means for limiting the transmissiontherethrough to a definite range of frequencies, and means cooperatingwith each duplex channel whereby the transmission of waves therethroughin one direction changes from eflicient transmitting to ineflicienttransmitting condition the channel for repeating waves in the otherdirection. I

In witness whereof, I hereunto subscribe my name this 20th day ofFebruary A. D., 1920.

JOHN MILLS.

